Aquaculture Engineering

Methods of plane tabling - Intersection

Unit 1- Surveying

Working operations

Methods of plane tabling

Intersection

Intersection is resorted to when the distance between the point and the instrument station is either too large or cannot be measured accurately due to some field conditions. The location of an object is determined by sighting the object from two plane table stations and drawing the rays. The intersection of these rays will give the position of the object. It is therefore very essential to have at least two instrument stations to locate any point. The distance between the two instrument stations is measured and plotted on the sheet to some scale. The line joining the two instrument stations is known as the “base line”.

The following is the procedure to locate the points by the method of intersection.

• Set the table at “A” level it and transfer the point “A” on to the sheet by way of plumbing fork. Clamp the table.
• With the help of the trough compass mark the north direction on the sheet.
• Pivoting the alidade about “a” sight it to “B”. Measure AB and plot it along ray to get “b”. The base line “ab” is thus drawn.
• Pivoting the alidade at “a” sight the details C,D,E etc., and draw corresponding rays.
• Shift the table at “B” and set it there. Orient the table roughly by compass and finally by back sighting “A”.
• Pivoting the alidade about “b” sight the details C, D, E etc., and draw the corresponding rays along the edge of the alidade to interest with the previously drawn rays in C, D, E etc., The position of the points are thus mapped by way of intersection.